Because of bad and just lazy writing the horde has done to much shitty stuff.
Can't forgive
But they can surely move past that as there's a new baddie of the month they'll have to team up to defeat it anyway
This world don't give us nothing. It be our lot to suffer... and our duty to fight back.
Its kinda sad how much wow has tried to hammer home that no, the the horde are bad afterall. As opposed to WC3 which tried to make all the bad stuff the fault of demons.
Thrall's horde was generally pretty decent. It had normal resources/territory conflict with the NE, but didn't do anything insane. Alliance and Horde disputes are more 'mundane' in this time, stuff you'd expect medieval countries to do. Both sides do kinda unfortunate stuff to the other, but the scale is relatively small.
The Garrosh came and amped up the conflict, dropping bombs, drinking the sha juice. But hey, maybe he just has a warped perception of the demon times?
Nah, WoD then has to prove that you gives orcs some freedom/power and they'll happily make roads of bones for you.
Okay so orcs are apparently bad in general, lets put a non-orc in charge. Vol'jin dies off quickly and we get sylvannus, who then does exactly what garrosh did.
No wonder Thrall and Saurfang are so sad, their dream is utterly ruined at this point.
Hyjal should be pretty Impregnable with all of the fortifications which would have been done to deal with the TH and the forelands, with all that prep and the whole NE fighting force and multiple wolf gods living there the chases the horde could ever push them out should be zero.
There can never be peace, just look at real life as an example.
At this point baddies of the month dealt less damage to the Alliance then the horde. I genuinely think that siding with "evil light" kinda like Yrel is more beneficial then helping horde defeat it at this point.
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Horde officially has no claims on Ashenvale since they traded it for Azshara. Leaving Ashenvale for them now is asinine and dangerous both. There can be peace as long as they leave it. Plus Darkshore is Alliance controlled canonically.
And if night elves move to Eatern Kingdoms then Blood elves and forsaken should move to Kalimdor. Enirely. Leaving Silvermoon and Lordaeron.
But they wont because its their ancestral land which they have all the rights to own so neither should night elves be forced to relocate.
Horde has all the resources they want with Azshara and their more recent new zones like Highmountain , Suramar and Zandalar. They have huge ports , rich land and abundance of resources. Horde has literally no need for Ashenvale other then just greed right now.
There can be peace when they settle down and stop trying to take whats not theirs, maybe try to be less genocidal too.
If you always run away...then you never have to fight. A rather fucked up philosophy.
The opposite would be the case, if the horde still was in Ashenvale she or at least one of her commanders (Maiev or Shandris) would have been there fighting for it.
It was stated by Blizzard that the Nightelves won Darkshore which would automatically lead to further battles in Ashenvale which, according to the mission table, never was fully taken by the horde.
The fact that all of these 3 where at Hyjal shows that all fighting was over at that time.